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MIDDLE CAMBRIAN. ile} 
the lamelle, marked by abundant parallel mica scales, pass directly through the nodule. 
In these the nodule is apparently due to the silicification of a portion of the stratified 
mud, which originally differed but slightly, if at all, from that which formed the mass 
of the surrounding shales. 
MODE OF OCCURRENCE OF THE SILICEOUS NODULES. 
The description of the mode of occurrence of the siliceous nodules is 
based largely on notes prepared by Dr. C. Willard Hayes, who mapped 
the areal geology of the region and studied the rocks with great care. I 
made a hurried trip to the region in which the nodules occur, in company 
with Dr. Cooper Curtice and Mr. 8. W. McCallie, of the Geological Survey 
of Alabama, in the summer of 1895. 
The shale from which the ‘‘cobbles” or siliceous nodules were derived 
is finely laminated, greenish, yellowish, or gray at the surface, and gener- 
erally bluish-black below drainage. They are found in several narrow 
bands, extending northeast and southwest, near the center of the Coosa 
Valley. These alternate with other bands of brown argillaceous shales and 
bands of interbedded shale and limestone. Although the evidence is far 
from conclusive, it appears probable that the cobble beds belong in the 
highest, the limestones in the intermediate, and the brown shales in the 
lowest division of the Coosa Valley formations. If this is the case, the 
cobble beds correspond with beds of greenish micaceous sandstone along 
the southeastern side of the valley. In some places these sandy beds can 
be traced directly into the cobbles through all intermediate gradations. 
The sandstone beds become thinner, the grains finer, and the silicification 
less uniform, resulting at first in thin plates slightly more resistant than 
the mass of the shale, then broad, thin lenses, and finally the flat ellipsoidal 
cobbles. Also, the cobble beds probably correspond with the shales on the 
northwest side of the valley, which carry thin plates of limestone and calca- 
reous nodules, the latter closely resembling the siliceous nodules in which 
the medusze are found. They vary considerably in the amount of calcare- 
ous matter which they contain, from nearly pure to highly siliceous 
limestone. The silica appears to be original and not secondary, due to 
replacement since the rock solidified. 
If the above correlation is correct, there is a marked change in the 
composition of contemporaneous deposits from southeast to northwest— 
from silicified micaceous sandstone, through argillo-siliceous shales contain- 
ing siliceous nodules to argillo-calcareous shales containing calcareo-siliceous 
